As wireless communication systems such as cellular telephone, satellite, and microwave communication systems become widely deployed and continue to attract a growing number of users, there is a pressing need to accommodate a large and variable number of communication subsystems transmitting a growing volume of data with a fixed resource such as a fixed channel bandwidth accommodating a fixed data packet size. Traditional communication system designs employing a fixed resource (e.g., a fixed data rate for each user) have become challenged to provide high, but flexible, data transmission rates in view of the rapidly growing customer base.
The third generation partnership project long term evolution (“3GPP LTE”) is the name generally used to describe an ongoing effort across the industry to improve the universal mobile telecommunications system (“UMTS”) for mobile communications. The improvements are being made to cope with continuing new requirements and the growing base of users, and higher data rates and higher system capacity requirements. Goals of this broadly based project include improving communication efficiency, lowering costs, improving services, making use of new spectrum opportunities, and achieving better integration with other open standards and backwards compatibility with some existing infrastructure that is compliant with earlier standards.
The wireless communication systems as described herein are applicable to, for instance, 3GPP LTE compatible wireless communication systems and of interest is an aspect of LTE referred to as “evolved UMTS Terrestrial Radio Access Network,” or “E-UTRAN” and also “UTRAN” communications systems. In E-UTRAN systems, the e-Node B may be, or is, connected directly to the access gateway (“aGW,” sometimes referred to as the services gateway, or “sGW”). Each Node B may be in radio contact with multiple types of user equipment (“UEs”, which generally include mobile transceivers or cellphones, although other devices such as fixed cellular phones, mobile web browsers, laptops, PDAs, MP3 players, and gaming devices with transceivers may also be UEs) via the radio Uu interface.
In the present discussion, particular attention is paid to enhancements presently being considered for Release 9 and Release 10 (sometimes referred to as “LTE Advanced”) of the 3GPP standards. These future evolutions of LTE will have additional requirements and demands for increased throughput. Although the discussion uses E-UTRAN as the primary example, the application is not limited to E-UTRAN, LTE or 3GPP systems. In general, E-UTRAN resources are assigned more or less temporarily by the network to one or more UEs by use of allocation tables, or more generally by use of a downlink resource assignment channel or physical downlink control channel (“PDCCH”). The PDCCH is used to allocate resources in other channels, including the physical downlink shared channel (“PDSCH”). LTE is a packet-based system and, therefore, there may not be a dedicated connection reserved for communication between a UE and the network. Users are generally scheduled on a shared channel every transmission time interval (“TTI”) by a Node B or an evolved Node B (“e-Node B”). A Node B or an e-Node B controls the communications between user equipment terminals in a cell served by the Node B or e-Node B. In general, one Node B or e-Node B serves each cell. A Node B or e-Node B may be referred to as a “base station.” Resources needed for data transfer are assigned either as one time assignments or in a persistent/semi-static way. The LTE, also referred to as 3.9G, generally supports a large number of users per cell with quasi-instantaneous access to radio resources in the active state. It is a design requirement that at least 200 users per cell should be supported in the active state for spectrum allocations up to 5 megahertz (“MHz”), and at least 400 users for a higher spectrum allocation.
In order to facilitate scheduling on the shared channel, the e-Node B transmits a resource allocation to a particular UE in a downlink channel PDCCH to the UE. The allocation information may be related to both uplink and downlink channels. The allocation information may include information about which resource blocks in the frequency domain are allocated to the scheduled user(s), the modulation and coding schemes to use, what the size of the transport block is, and the like.
The lowest layer of communication in the UTRAN or e-UTRAN system, Layer 1, is implemented by the Physical Layer (“PHY”) in the UE and in the Node B or e-Node B and the PHY performs the physical transport of the packets between them over the air interface using radio frequency signals. In order to ensure a transmitted packet was received, an automatic retransmit request (“ARQ”) and a hybrid automatic retransmit request (“HARQ”) approach is provided. Thus, whenever the UE receives packets through one of several downlink channels, including dedicated channels and shared channels, the UE performs a communications error check on the received packets, typically a Cyclic Redundancy Check (“CRC”), and in a later sub-frame following the reception of the packets, transmits a response on the uplink to the e-Node B or base station. The response is either an Acknowledge (“ACK”) or a Not Acknowledged (“NACK”) message. If the response is a NACK, the e-Node B automatically retransmits the packets in a later sub-frame on the downlink (“DL”). In the same manner, any uplink (“UL”) transmission from the UE to the e-Node B is responded to, at a specific sub-frame later in time, by a NACK/ACK message on the DL channel to complete the HARQ. In this manner, the packet communications system remains robust with a low latency time and fast turnaround time.
Many types of UEs may be accommodated by the UTRAN or e-UTRAN. One type of UE service that is presently proposed to be supported in UTRAN and e-UTRAN systems is a UE that includes support for MIMO transmissions. A MIMO UE may have a plurality of antennas and receivers, instead of only one. For example, a MIMO UE may have 2, 4 or more antennas and receivers. Also a transceiving device such as a base station transmits the message for a UE on more than one antenna. By providing multiple pathways for a transmitted message, the likelihood a transmitted message is received without error is increased, and the robustness and coverage of the system is therefore increased.
In single user MIMO (“SU-MIMO”) a high rate signal at the transmitter can be split into multiple lower rate signals transmitted simultaneously to a receiver. If the receiver has an array of receive antennas and the signals are sufficiently spatially separated, the receiver can form parallel input streams which can then be combined, thus increasing system throughput while maintaining a lower signaling rate. The applications for the embodiments described herein are directed more specifically to multi-user MIMO (“MU-MIMO”). In MU-MIMO, a transmitter simultaneously transmits different signals over multiple antennas to different receivers also having multiple antennas. Because the signals for a particular UE receiver are spatially multiplexed and spatially separated from the other signals, the receivers can all receive their signals at the same time, thus increasing system throughput.
In implementing a MIMO scheme as proposed in the prior art, the eNB needs to be able to reliably communicate to the UE without undue interference caused by transmissions to other UEs spatially multiplexed on the same radio resources. Present 3GPP standards define MU-MIMO signaling for up to four spatially multiplexed users. In one known approach, each UE is given a separate downlink (“DL”) grant. A vector or index indicates the pre-coding to be used for the UE's own transmission. This index is referred to in the specifications as the “pre-coding matrix index” (“PMI”). The index points into a predefined table of pre-coding vectors that, as required by the present standards, is known to both the UEs and the eNBs.
The present approach to MU-MIMO schemes attempts to make the transmission used for spatially multiplexed transceivers completely orthogonal. In the Release 8 standards for 3GPP, pre-coding vectors to be used for MU-MIMO are taken from a predefined codebook. If multi-user orthogonality were in fact accomplished with the codebook, the UEs could operate without inter-user interference. This would require, at least, extremely fine pre-coding granularity and a huge pre-coding vector codebook, which is impractical. In a practical system, the multi-user pre-coding can never be perfect and some inter-user interference will be present.
In the systems of the prior art, no information is signaled to one UE about other UE's pre-coding vectors. Without this information, a UE cannot actively suppress the inter-user interference that is left in the received signals. Because the wireless radio channels and the pre-coding vectors do not match perfectly, there is always some remaining multi-user interference in the signals received at the UE.
It is well known that if the UEs in a single cell or multiple cell multiple user group would know the codebook vectors or pre-coding matrix indices (“PMIs”) used by the other UEs, the interference due to inter-user interference can be significantly mitigated. The number of interferers that can be canceled out depends on the spatial degrees of freedom. That is, the number of receivers and antennas available in the UE determines how many spatial interferers can be eliminated in a received signal. For example, assuming that one stream is transmitted to a UE, a UE with two receiver antennas can eliminate one spatial interferer, while a UE with four receiver antennas can eliminate up to three spatial interferers. In order to eliminate the interference, the system must signal the codebook vectors or PMIs of the interfering UEs to the other UEs as well. One known solution to this signaling requirement is to signal them all in the same DL grant from the eNB. This approach has been proposed by several companies participating in developing the 3GPP standards. However, this approach would also require significant increases in signal payload size of the DL grant and this will use up the available resources in the PDCCH. The increase in payload size of the DL grants is particularly significant when multiple UEs are spatially multiplexed together, and/or the code book or PMI is large, that is, if the PMI signaling requires a large number of bits.
A need thus exists for methods and apparatus to efficiently support the MU-MIMO capability for UEs in an over the air interface communications system, with efficient methods to eliminate inter-user interference, without the disadvantages of the known approaches.